The present invention relates to antennas and, more specifically, to a stacked, multi-band, look-through antenna structure with a small frequency separation between operating bands.
Applications requiring transmission and/or reception of radio frequency (RF) signals, typically in the microwave or millimeter wave bands, are numerous. Such applications include radar systems, satellite communications systems, aircraft altimeter and guidance systems, friend or foe (FOF) identification systems and ground reconnaissance mapping systems. Each of these applications requires transmitting RF energy through free space. Each system, therefore, also requires an antenna for receiving or radiating this RF energy to or from free space, the antenna acting as a transition between a wave guiding structure (i.e., a transmission line or the like) and free space. Many types of antennas exist and are well known to those skilled in the art, each of these known antennas having both advantages and disadvantages.
In many systems, both commercial and military, multiple systems or applications require simultaneous transmission and reception of RF signals. For example, aircraft typically have radar systems, ground communications, and air-to-air communications systems. In these systems, at least one antenna is used by each system. A problem arises when limited surface space, known as real estate, is available for deploying the necessary antennas. This is often the case with aircraft and almost always a problem with satellites.
In general, it is difficult to implement multiple antennas in close proximity to one another because of interference and crosstalk problems. To overcome the real estate problem, attempts have been made to combine more than one function and/or frequency of operation into a single antenna structure without incurring the aforementioned crosstalk and interference problems.
U.S. Pat. No. 4,864,314 for DUAL BAND ANTENNAS WITH A MICROSTRIP ARRAY MOUNTED ATOP A SLOT ARRAY, issued to Kevin J. Bond, teaches one such antenna. BOND discloses a primary slotted array antenna operated in the 10 GHz frequency range with a secondary antenna mounted in front of the primary antenna. This front antenna is designed to operate in the 1 GHz range and be essentially transparent to the 10 GHz signal from the rear antenna.
In contradistinction, the stacked, multi-band antenna of the present invention is designed to allow a much closer spacing of operating frequency bands, typically on the order of 4:1 not the 10:1 frequency ratio of the BOND antenna. In addition to the critical upper and lower operating frequency band separation, the BOND antenna is good for only single linear polarization of the radiated field wave, while the inventive antenna may be used in dual linear polarization and circular polarization modes.
Another approach to a multi-band antenna is disclosed in U.S. Pat. No. 5,485,167 for MULTI-FREQUENCY BAND PHASE-ARRAY ANTENNA USING MULTIPLE LAYERED DIPOLE ARRAYS; issued to Nam S. Wong, et al. In the WONG, et al. system, several layers of dipole pair arrays, each tuned to a different frequency band, are stacked relative to each other in positions to form frequency selective surfaces. The highest frequency array is in front of the next lowest array, and so forth. Due to the frequency-selective property of the arrays, incident high frequency signals are absorbed by the highest frequency array. However, low frequency signals experience only a minimal loss in passing through the higher frequency, upper antenna array layers. This results in acceptable performance of the lower frequency antenna array layers.
The stacked, multi-band antenna of the instant invention, however, places the highest frequency antenna elements at the bottom of the stack with the lower frequency elements in front. In fact, the rear, high frequency element may serve as a ground plane for the front, lower frequency antenna. There are three major differences between the inventive antenna and that of WONG, et al. First, the arrangement of the frequency layers is different. The inventive antenna has the lowest frequency band antenna layer at the outermost layer, but WONG, et al. put the highest frequency antenna at the outermost layer. The second difference is that WONG, et al. requires a xe2x80x9cwirescreenxe2x80x9d ground plane for every layer of antenna. In other words, there must be five ground plane screens if there are five frequency bands of operations. In the inventive antenna, only one ground plane is required for two or more layers (i.e., frequency bands of operation). The third point of difference is that in the WONG, et al. antenna, the polarization of all layers may only be linear. In addition to this limitation, the two junction layers must be transposed linearly polarized antennas. That is, if layer number two is an X-polarized antenna element, then layers number one and three must be Y-polarized antenna elements. The inventive antenna has not such constraint on the polarization of individual layers. For example, it can simultaneously perform as single or dual linear polarized antennas or as a circularly polarized antenna.
Still another approach to a multi-band antenna is disclosed in U.S. Pat. No. 5,982,339 for ANTENNA SYSTEM UTILIZING A FREQUENCY SELECTIVE SURFACE, issued to Farzin Lalezari, et al. LALEZARI, et al. use antenna elements having frequency selective surfaces (FSS) aligned in front of one another. The FSS of the front-most antenna element is designed to absorb a high frequency signal to which the antenna element is responsive, while making the elements appear transparent to lower frequencies to which one or more lower (rearward) antenna elements are tuned.
In accordance with the present invention there is provided a stacked, multi-band antenna system consisting of a low-frequency, forward portion and a gridded, rear portion designed for operation at a higher frequency. Both front and rear antenna sections may share a common ground plane or the rear antenna section may form a ground plane for the front antenna. Typically, the front antenna is a relatively narrow-band, gridded, bow-tie dipole and the rear antenna is a wide-band dipole or slot element. Additional frequency bands may be designed into the inventive system by adding additional dipole or similar antenna elements either in front of, between, or behind the front and rear antennas. By properly choosing element sizes and spacings, a frequency band ratio of as little as 4:1 can be accommodated.
It is therefore an object of the invention to provide a stacked, multi-band antenna system having a small ratio between operating frequency bands.
It is another object of the invention to provide a stacked, multi-band antenna wherein a high-frequency portion of the antenna is located being and in line with a low frequency portion of the antenna.
It is a still further object of the invention to provide a stacked, multi-band antenna where a low-frequency, front portion of the antenna may use the rearward, high-frequency portion of the antenna as a ground plane.
It is yet another object of the invention to provide a stacked, multi-band antenna wherein a front portion, a rear portion, or both portions of the antenna system are arrays.
It is a still further object of the invention to provide a stacked, multi-band antenna in which at least one of the antenna arrays is steerable.
It is an additional object of the invention to provide a stacked, multi-band antenna that may be combined into an antenna array.
One object of the invention is a stacked, multi-band see-through antenna, comprising a ground plane, and a first radiating element spaced a predetermined distance from the ground plane along a transmission/reception direction, wherein the first radiating element is tuned to a first operating frequency. The invention further comprises a second radiating element disposed along the transmission/reception direction and intermediate the first radiating element and the ground plane. The second radiating element is tuned to a second operating frequency that is greater than and in the range of four times the first operating frequency.
Another object is a stacked, multi-band see-through antenna, wherein the transmission/reception direction is substantially perpendicular to the ground plane.
An additional object includes a stacked, multi-band see-through antenna, further comprising RF signal feed means operatively connected to both the first and the second radiating elements.
Yet a further object is the stacked, multi-band see-through antenna, wherein the RF signal feed means comprises a first RF signal feed means operatively connected to the first radiating element and a second, independent RF signal feed means operatively connected to the second radiating element. And, the stacked, multi-band see-through antenna, wherein the RF signal feed means comprises a common RF signal feed means operatively connected to both the first radiating member and the second radiating element.
Another object is the stacked, multi-band see-through antenna, wherein the RF signal feed means comprises at least one from a group of devices: balun, splitter and filter.
A further object is the stacked, multi-band see-through antenna, wherein the first radiating element comprises a dipole array. Alternatively, the stacked, multi-band see-through antenna, wherein the second radiating element comprises a slotted array.
An additional object is for the stacked, multi-band see-through antenna, further comprising a first spacing means disposed between the ground plane and the second radiating element for supporting the second radiating element a predetermined distance from the ground plane. Also, for a second spacing means disposed between the first and the second radiating elements for supporting the first radiating element a predetermined distance from the second radiating element.
Another object is the stacked, multi-band see-through antenna, wherein the second radiating element is angularly disposed in relation to the first radiating element. Angularly disposed refers to the orientation of certain wires of the layers being positioned about 45 degrees relationship from the underlying layer in order to accommodate a dual band system.
Yet a further object is a stacked, multi-band see-through antenna, further comprising one or more radiating elements interspersed about said first and second radiating elements. As defined herein, interspersed refers to positioning one or more additional radiating elements above, below or between either of the first and second radiating elements, thus forming stackable layers of radiating elements.
An object of the invention is a stacked, multi-band, see-through antenna, comprising a ground plane having a front and a rear surface, with a first spacer means having a front and a rear surface, the rear surface of the first spacer means being disposed on the front surface of the ground plane. There is a first substantially planar radiating element adapted for operation at a first frequency and having a front and a rear surface, the rear surface of the first radiating element being disposed on the front surface of the first spacer means. A second spacer means having a front and a rear surface, the rear surface of the second spacer means being disposed on the front surface of the first radiating element. There is a substantially planar second radiating element adapted for operation at a second, predetermined frequency, the second frequency of operation being lower than the first frequency of operation, the second radiating element having a front surface and a rear surface, with the rear surface of the second radiating element being disposed on the front surface of the second spacer means.
And yet another embodiment is the stacked, multi-band, see-through antenna, wherein the first and the second spacer means comprise foam.
An object includes the stacked, multi-band, see-through antenna, wherein the first operating frequency and the second operating frequency are in a ratio of approximately 4:1.
Yet a further object, the stacked, multi-band, see-through antenna, further comprising a resonant cavity proximate the front surface of the ground plane.
Another object is the stacked, multi-band, see-through antenna, further comprising signal feed means adapted to feed an RF signal to the first radiating element and to the second radiating element.
An additional object is the stacked, multi-band, see-through antenna, wherein the first radiating element comprises a slot array. Also, wherein the second radiating element comprises a dipole array.
A final object of the invention is the multi-band, see-through antenna, wherein the first frequency of operation comprises an S-band frequency in the range of approximately 2.0-4.0 GHz.